1. Field of the Invention
The present invention relates to a device for correcting the deflection effect due to a variation of the focusing voltage in a trichromatic cathode ray tube with in line cathodes.
2. Discussion of the Background
It has been discovered that in trichromatic cathode ray tubes, of the type with a gun having a prefocusing lens and a main lens, that is to say having generally five or six grids in all, when the focusing voltage is varied, passing for example from 8 to 10 kV, the position of the spots is modified corresponding to the red and blue colors, for illuminating the same luminophores. This phenomenon has been discovered during alignment tests of the gun (known as Focus Raster Alignment Test or FRAT). This modification of the position of the spots has a unfavorable action on the convergence.
To remedy this effect, the form of grid 2 is modified, which is then called "G2 with fixed FRAT", by stamping its central part. Such stamping causes inevitable deformation of the flat surfaces of the grid acting unfavorably on the astigmatism of the tube, which requires increasing the focusing voltage required for counterbalancing this phenomenon.
The present invention provides a device for correcting the above-mentioned effect, not requiring modification of G2, which remains flat, which device is simple and inexpensive to put into practice.